Water produced from crude oil and natural gas production contains varying residual concentrations of crude oil, natural gas condensates and solids. Water and oil mixtures also result from the water washing of refined petroleum fractions following generally known refining processes, as well as from drainage from the various equipment used in such processes. These contaminants must be removed before the water suspension component can be either used in secondary recovery operations or safely discarded.
Small droplets of oil are suspended in the produced water and held there by mechanical, chemical and electrical forces. The amount of oil contained in the untreated produced water in most systems will vary from an average low of about 5.0 ppm to an average high of about 2,000 ppm. In some water systems, oil contents as high as 20,000 ppm (2%) have been observed.
The oil particles in the untreated produced water will usually vary in size from 1 to about 1,000 .mu.m with most of the oil particles ranging between 10 and 100 .mu.m in diameter. Various methods have been suggested for use in removing oil from produced water, including methods based upon gravity separation of lighter oil droplets from the water or gas flotation of the oil droplets. The method and apparatus of the present invention comprise an improvement to the gas flotation technique.
In gas flotation units, large quantities of gas bubbles are injected into the water stream. These bubbles attach to the oil droplets suspended in the stream and cause them to rise to the water's surface as froth. The efficiency of the gas bubble/oil droplet collision increases as the bubble diameter approaches the oil droplet diameter. Therefore, smaller gas bubbles are more effective in removing oil droplets from water, especially for small oil droplets, i.e., droplets less than 10 microns in diameter.
Two distinct types of flotation units are commonly used, which are distinguished by the method employed in producing the gas bubbles needed to contact the water. These are the dissolved-gas units and dispersed-gas units. Dissolved-gas units take a portion of the treated water effluent and saturate the water with a gas, such as natural gas, or air, in a contactor. The higher the pressure the treated water is subjected to, the more gas can be dissolved in the water. Generally, dissolved-gas units utilize a contact pressure of about 20 to 40 psig, with about 20% to 50% of the treated water recirculated for contact with the gas. The gas saturated water is then injected into the flotation chamber where the gas breaks out of solution as small diameter bubbles when the flow enters the lower pressure chamber. In the dispersed-gas units gas bubbles are mechanically dispersed in the total stream either by means of an inductor device or by a vortex set up by mechanical rotors. Most dispersed-gas units contain three or four cells, where bulk water flow moves in series from one cell to the other by underflow baffles.
There are several disadvantages inherent in both the dissolved-gas and dispersed-gas flotation systems. The first is that both systems rely on a recycling of the produced gas, which leads to the gas eventually coming to equilibrium with the water, making it ineffective for use in volatile hydrocarbon stripping. In the dispersed-gas system there is the additional problem associated with the use of high shear pumps or mixers which tend to shear the oil drops into smaller and more difficult to remove droplets. In the dissolved-gas system the bubbles created are very small (i.e., &lt;100 micron). In oil flotation technology, smaller bubbles have more effective collisions with the smaller oil drops. Therefore the small bubbles achieved with the dissolved-gas system is very effective in contacting oil. However, these bubbles are so small as to have two disadvantages. First, they take several minutes to float the oil to the top of the water. Second, their rise velocity is too slow to rise against a downward liquid flow. Thus, they limit the liquid flow rate at which the equipment can process produced water for oil removal. These very small bubbles require more than 10 minutes to rise to the surface in a quiescent tank. In a tank with flow through it the bubble is unable to rise against any downward velocity. We have found in accordance with the present invention that certain sintered spargers are optimally suited specifically to oil flotation in that they produce bubbles in an optimum size range for oil removal from produced water, i.e., bubbles having diameters .gtoreq.100 microns and .ltoreq.1000 microns, which are smaller than induced flotation bubbles and therefore more effective, faster, and thorough, while remaining large enough to rise to the surface of conventional flotation cells in less than 1 minute even when there is a downward hydraulic flow imposed by the flow through the vessel.
U.S. Pat. No. 2,766,203 (Brown et al) ("the '203 patent") describes a water purification process and apparatus using the dissolved-gas method to introduce gas bubbles into the system. The system described uses a recycle stream of water from the flotation chamber for the injection of the gas. Brown states that only through the partial dissolution of flotation gas in a recycle water stream and subsequent depressuring and dispersion can bubble sizes in the range of 10.sup.-2 to 10.sup.-4 mm (10 to 0.1 Micron) and 0.5 to 5 mm (500 to 5000 microns) be obtained, and efficient results obtained.
The use of sparging tubes in solid/liquid separation systems is known, as for example in U.S. Pat. No. 5,122,261 (Hollingsworth), which discloses the use of sintered metal tubes to introduce air into mineral pulp flotation columns. That patent also describes plugging that is anticipated with the use of sintered sparging tubes in pulp flotation.
Typical dispersed gas flotation systems have complex internal components, such as in U.S. Pat. No. 5,158,678 (Broussard), which are necessary to introduce and/or disperse the gas bubbles into the system. Further, these systems require additional external apparatus for secondary separation and recycle of the gas, oil, and water streams, as shown in the '203 patent.
It is therefore an object of the present invention to provide a means for purifying produced water which does not rely on high shear pumps, mixers, or gas saturated water to produce gas bubbles, and which reduces the number of components required for the flotation.
It is a further object of this invention to develop a system for generating optimally sized diameter gas bubbles in which the produced gas is not recycled through the system and in which contact times of about 1 minute/cell are adequate for the gas to rise to the surface and disengage from the liquid.
It is an advantage of the present invention that the gas induced into the system is used only once, and then recovered for alternative use.
It is also an advantage of the present invention that recycle loops for removed water, oil, and gas are not required. It is a further advantage that the sparger system can be applied to existing systems by adding sparger tubes to the existing system to generate bubbles, thereby allowing the existing mechanical bubble generation system to be removed or shut down.